1. Field of the Invention
The invention relates to a method of pupil segmentation in a digital image of a vertebrate eye. The term “vertebrate” shall be understood to include humans.
2. Description of Background Art
A method of this type is used in the context of biometric identification systems based on iris analysis.
A biometric personal identification system based on iris analysis has been described by John G. Daugman in U.S. Pat. No. 5,291,560 and in J. G. Daugman: “How Iris Recognition Works”, IEEE Transactions on Circuits and Systems for Video Technology, Vol. 14, no. 1, pp. 21-30, January 2004.
Such identification systems take advantage of the fact that the iris of the eye of an individual, which may be a human or a vertebrate, has a characteristic pattern that is unique for that particular individual, so that an iris analysis may be used for uniquely identifying the individual. To that end, an image of an eye of the individual is captured with a digital camera, and image processing algorithms are used for recognizing the pupil and the iris of the eye in the digital image. Then, the iris pattern is normalized so as to compensate the effect of varying dilation or contraction of pupil, and a filter procedure is employed for transforming the normalized image of the iris into a digital code, a so-called iris code, that is unique to the individual and may therefore be used for identification purposes.
Once an iris code of an individual has been created and stored, that individual may be identified by capturing again an image of its eye, creating an iris code on the basis of the new image, and checking the iris code thus obtained against the code that had been stored previously.
EP 0 821 912 B1 describes a pupil segmentation method that is based on the consideration that the pupil of an eye is an area that has a low density and low contrast. The pupil region in the image is therefore determined by extracting a region having a surface area within a range of predetermined threshold values. The result is a pupil area which has an irregularly shaped boundary. This boundary is then approximated by an ellipse which is considered as the boundary of the pupil.
US 2009/0208064 A1 describes a pupil segmentation method wherein a pupil region which may have an irregular shape is determined by extracting a region having an intensity that is lower than a certain threshold and then computing the convex hull of that region.